Spray a 12-cup muffin tin with nonstick cooking spray.
In a medium mixing bowl, whisk together the flour, cornstarch, baking powder, baking soda, and salt. Set aside.
In a separate bowl, beat together the sugar, brown sugar, and oil until smooth and well combined. Add the eggs one at a time, beating between each addition. Beat in the milk and vanilla.
Add the dry ingredients to the wet ingredients and mix until just combined.
Fold in the mini chocolate chips and sprinkles until well combined.
Fill each cup of the muffin pan 2/3 of the way full. Sprinkle additional chocolate chips and sprinkles on top of each muffin, if desired.
Bake 18-20 minutes or until a toothpick inserted into the center comes out clean.